“Fishing” Polymer Brushes on Single-Walled Carbon Nanotubes by in-Situ Free Radical Polymerization in a Poor Solvent Guiquan Guo, ² Dong Yang, ² Changchun Wang,* ,² and Shu Yang ‡ Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science, Fudan UniVersity, Shanghai 200433, China, and Department of Materials Science and Engineering, UniVersity of PennsylVania, 3231 Walnut Street, Philadelphia, PennsylVania 19104 ReceiVed July 28, 2006; ReVised Manuscript ReceiVed October 30, 2006 ABSTRACT: Single-walled carbon nanotubes (SWNTs) exhibit unique thermal and electrical conductivity and high mechanical strength. The ability to effectively functionalize the SWNT surface and control their dispersion in a polymer matrix will be crucial to exploit their physical properties in nanocomposites. Here we report the first example of grafting polymers onto SWNTs in a poor solvent through a “fishing” process. The SWNTs act as “fishhooks”, and the “living” polymer radicals are “fish”, which are enthalpically favored to absorb onto the surface of SWNTs and continue to propagate until all the active sites are consumed. We demonstrate to graft 1 g of SWNTs with ∼20 wt % poly(methyl methacrylate) in 100 mL of methanol/water (1/4 by volume) with monomer concentration as low as 5 mg/mL and monomer/SWNTs ratio of 0.5:1 by weight. The structure of functionalized SWNTs was characterized by Raman spectroscopy, UV-vis spectroscopy, HRTEM, and AFM. The polymer grafting method we described is fundamentally different from the reported approaches, where the polymerization takes place either solvent-free or in a good solvent but requires much higher monomer concentrations and monomer/SWNTs ratios. The promise of synthesizing gram-scale functionalized SWNTs from a wide range of polymers in a small volume of solvent may greatly improve our ability to engineer novel SWNT composites. Introduction Isolated single-walled carbon nanotubes (SWNTs) exhibit unique thermal, mechanical, and electrical properties and are of great interest in exploitation of their physical properties in nanocomposites for applications, including molecular electron- ics, sensors, field emission displays, and ultrahigh-strength materials. 1-3 However, the intertube attraction due to van der Waals interaction is very strong, ∼40 k B T/nm, 4,5 resulting in poor dispersion of SWNT in common organic solvents and polymeric matrices, which significantly hinders the high loadings of SWNTs in composites. To improve the dispersal of SWNTs in solvents or polymers with a sufficiently high concentration, various surface functionalization methods have been investi- gated, including physical bonding (e.g., coating SWNTs with surfactants) and chemical bonding. Although the physical bonding is simple and straightforward and has the advantage of maintaining the nanotube structure and its electronic proper- ties, the choices of surfactants and polymers are rather limited. By covalent bonding, a wide range of chemical functionalities, including long alkyl chains, aromatic groups, hydrophilic and hydrophobic polymers, and biomolecules, can be readily at- tached to SWNTs either by direct radical reaction with SWNTs or via carboxylic groups, 6-11 which can be further functionalized through esterification or amidation to graft hydroxyl-terminated oligomers and polymers, 12-16 respectively. Since high grafting density is necessary for high solubility of SWNTs, grafting high-molecular-weight polymer chains onto the surface of the SWNTs holds promise to improve the interfacial adhesion and the load transfer efficiency. This can be achieved through either “graft to” or “graft from” approach. 17 The “graft to” method involves chemical bonding of preformed, end-functionalized polymers to a reactive surface. The “graft from” method involves immobilization of initiators on the substrate, followed by in-situ surface-initiated polymerization to generate tethered polymer chains. It has been demonstrated to grow polymers up to 70 wt % from carbon nanotubes using a “graft from” strategy. 18-20 This is in sharp contrast to the considerably lower level (<15 wt %) of polymers grafted on nanotubes using the “graft to” approach, especially in the case of a high-molecular-weight, end-functionalized polymers. 21 This is because the diffusion of long polymer chains to the available reactive sites could be sheltered by the existing brushes and the reaction is rather heterogeneous in the “graft to” approach. Alternatively, a relatively high degree of functionalization, 20-30 wt % of organic molecules, has been achieved in a solvent-free system, 22 although partial deroping of the bundles is necessary before the reaction, for example by mechanical stirring. Qin et al. 23,24 have reported an effective method that grafts ∼40 wt % of polymers on as-prepared SWNTs by in- situ radical polymerization in a good solvent of the polymers. It was proposed that propagating polymer radicals were co- valently bound to SWNT bundles and the continued attachment of polymer chains deroped the pristine SWNTs to single tubes ² Fudan University. ‡ University of Pennsylvania. * Corresponding author: Fax 86-21-65640291, Tel 86-21-65642385, e-mail ccwang@fudan.edu.cn. Table 1. Grafting Efficiency of PMMA on SWNTs Dependence on Reaction Time a sample code reaction time (h) solubility in ethyl acetate (mg/mL) grafting amount of PMMA, b wt % PMMA-g-SWNTs-1 0.5 0.01 2.5 PMMA-g-SWNTs-2 1 0.15 19 PMMA-g-SWNTs-3 2 0.26 25 PMMA-g-SWNTs-4 3 0.28 23 PMMA-g-SWNTs-5 10 0.27 27 a Polymerization recipe: 5 g of MMA, 0.1 g of SWNTs, and 0.05 g of AIBN in 100 mL of methanol. Reaction temperature, T ) 60 °C. b This result was according to TGA measurement. All samples were heated from 50 to 800 °C at a heating rate of 10 °C/min under nitrogen flow (20 mL/min). 9035 Macromolecules 2006, 39, 9035-9040 10.1021/ma061715a CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2006